A patient presents with substernal chest pain that worsens while lying down but improves when leaning forward. What is the likely diagnosis?

The presentation of substernal chest pain that worsens while lying down and improves when leaning forward is characteristic of pericarditis. This condition involves inflammation of the pericardium, the fibrous sac surrounding the heart. One of the hallmark features of pericarditis is the positional nature of the pain; it is typically exacerbated by lying flat due to increased pressure on the inflamed pericardium and often alleviated by leaning forward, which reduces this pressure.

In contrast, myocardial infarction typically presents with chest pain that is not significantly influenced by body position and is often described as a pressure-like sensation. Pleuritis, or pleurisy, can cause sharp pain that worsens with breathing or coughing but does not typically improve by leaning forward. Gastroesophageal reflux disease can cause chest discomfort that may mimic cardiac pain, but it is usually related to meals or certain positions and does not have the same definite positional relief experienced in pericarditis.